The present invention generally pertains to a method and apparatus for monitoring fetal cerebral oxygenation during childbirth. More particularly, the present invention relates to a method and apparatus for monitoring fetal cerebral oxygenation using near-infrared spectroscopy.
As is well known, oxygen is transferred from the maternal blood to the fetal circulation through the placenta. During uterine contractions, increased pressure on the placenta impedes blood flow and interrupts the supply of oxygen to the fetus to a degree proportional to the duration, intensity, and frequency of the contractions. The fetus has a number of protective mechanisms including a special form of hemoglobin, with its dissociation curve shifted to the left, which binds more oxygen at lower partial pressures. Also, when the oxygen supply is cut off, the fetal metabolism continues to produce chemical energy (ATP) by anaerobically converting glucose to lactatexe2x80x94xe2x80x9cglycolysis.xe2x80x9d (Red cells themselves do not use oxygen but get all their energy this way.) Reestablishment of blood flow provides the oxygen to metabolize the excess lactate and return cells to normal metabolism.
Despite these protective measures, if the fetus is compromised by a sustained interruption of the blood supply or when the normal placental vasculature is impaired, profound fetal hypoxia can result in permanent brain damage or death. If fetal hypoxia is detected early enough, an operative delivery can be initiated to prevent or reduce brain damage. There are presently no good ways to monitor fetal oxygenation distress.
The fetal heart rate slows in response to low oxygenation (opposite to adults) and may be monitored either externally using Doppler ultrasound and other methods, or directly by hooking a small electrode into the fetal scalp. The latter is more reliable and is becoming more commonly used. Late slowing of the heart rate (decelerations) during each uterine contraction are thought to result from uteroplacental insufficiency and reflect inadequate fetal oxygenation. While very sensitive, the technique has such low specificity that it may lead to unjustified operative deliveries.
Because fetal cerebral oxygenation is not now directly measurable, consideration has been given to measuring the fetal scalp oxygenation in the hope it will provide some informationxe2x80x94even though its relationship to what is going on in central and cerebral oxygenation remains highly controversial. However, even though some of the fetal scalp is accessible early, the direct measurement of scalp oxygenation has many practical problems.
Blood samples are occasionally taken from the scalp for analysis, but this has the disadvantage of limited availability, prerequisites for use, and invasive nature. Moreover, however accurate, sampling provides only intermittent information about the very dynamically changing condition of the fetus during labor.
Continuous monitoring of blood pH (an indication of oxygenation) in the subcutaneous space of the fetal scalp through a hollow spiral needle has been described but is expensive and not used outside of clinical research centers.
Transcutaneous oxygen tension (tcPO2) using a Clark-type electrode is possible but requires a tight, dry seal between the scalp and the surface of the electrode. Other disadvantages include the need to constantly heat the skin under the electrode in order to increase oxygen diffusion through the skin, the slow response time of the electrodes, the dependence on the measurement on skin blood flow, and the inherent long-term drift of the tcPO2 electrodes.
Recently, in vivo near-infrared spectroscopy (NIRS) has shown considerable promise for noninvasive, direct monitoring of fetal and neonatal cerebral tissue oxygenation. The basis of the methodology is the surprising translucence of the body to near-infrared photons having wavelengths between about 700 and 1100 nm. The light is easily seen transmitted through thin body parts (cheek, ear, fingers, etc.) and a xe2x80x9cback-scatteredxe2x80x9d halo of reflected light can be observed from all, including thick tissues, and invisible near-infrared light is far more easily transmitted. The very long (random and tortuous) paths taken by the photons makes them exceedingly sensitive to the optical properties of tissue and, in particular, the concentration of hemoglobin molecules and the average amount of oxygen they are carrying.
Measuring available oxygen is very useful. At any moment, the body""s total supply of oxygen is only about one gram (just enough to last about four minutes), which is bound to the hemoglobin in circulating blood. Obviously, maintenance of the oxygen supply is crucial to avoid irreversible tissue death. The length of time that cells can survive following interruption of oxygen depends on the type of tissue. Brain neurons are unrecoverable after only several minutes.
Measurement of just how much oxygen hemoglobin is carrying is made possible because the infrared (and visible xe2x80x9ccolorxe2x80x9d) absorption spectrum of hemoglobin is strongly dependent on its oxygen saturation. Arterial blood leaving the heart gets its bright red color from hemoglobin that is nearly saturated with oxygen absorbed in the lungs. The hemoglobin of venous blood has given up much of its oxygen to tissue metabolism and turned dark and bluish.
The average amount of oxygen carried by hemoglobin molecules is expressed as the percent of xe2x80x9csaturation.xe2x80x9d Thus, hemoglobin in arterial blood, having just visited the lungs, is nearly 100% saturated. Many of the hemoglobin molecules in the venous blood will have given up this oxygen to the cells of the body and the average saturation may fall to half (50%) or less.
The nomenclature is mixed, but a hemoglobin molecule that has oxygen attached to all four binding sites is called xe2x80x9coxy-hemoglobin.xe2x80x9d Hemoglobin without any oxygen attached is called xe2x80x9cdeoxy-hemoglobin.xe2x80x9d Individual hemoglobin molecules will be in one state or the other, since it is very unlikely that any hemoglobin molecule will have only partial filling of its four oxygen-binding sites.
NIRS technology uses a source of near-infrared light to send photons into the skin over the organ of interest. After being scattered about inside the body, some photons survive to return and exit the skin. A detector at some nearby point measures their flux compared to the injected flux (xe2x80x9creflectancexe2x80x9d). Reflectance is defined as the number of returning photons per unit area per photon injected and is essentially an exponentially decreasing function of the effective absorption coefficient xcexceff=(3xcexcaxcexcsxe2x80x2) and the distance between source and detector.
Both the light source (typically a light emitting diode or tungsten lamp) and the detector (typically a silicon photodiode or photomultiplier) are called xe2x80x9coptodes.xe2x80x9d By doing this measurement at several wavelengths, one can infer the spectral absorptance, and hence estimate the concentration of hemoglobin (and hence blood) in the organ and its average oxygenation.
The xe2x80x9cpulse oximeterxe2x80x9d was the first successful device based on measuring infrared spectral reflectance, and is very widely used. It is an extension of the infrared heart rate monitors that can be clipped to the earlobe or finger and detect the change of light absorption as arteries in the light path expand and contract in response to pressure from heart muscle contractions. By detecting the change of absorption at two or more wavelengths, pulse oximeters can measure the average hemoglobin oxygen saturation in the pulsing arteries. By their nature, pulse oximeters are insensitive to the oxygenation of capillary and venous bloodxe2x80x94or, for that matter, arterial blood that is remote from the skin surface. Since the small changes in absorption are easily swamped by changes in optode spacing, pulse oximeters are very sensitive to patient motion.
Although manufacturers are attempting to adapt pulse oximeters to measure oxygenation of the fetal brain during birth, they are not well suited for this special application. Even if they can be made to reliably measure arterial oxygen saturation in the fetal scalp in the presence of massive subject motion, the blood flow may be so low that the brain is oxygen-starved while the arteries in the scalp are 95% oxygen saturated.
A more recent application of NIRS technology is typified by the Cerebral Oximeter (INVOS 3100) made by Somanetics Corporation of Troy, Mich., which currently is the only FDA-approved NIRS medical device that is sensitive to deep arterial and venous blood oxygenation. The theory behind the Somanetics instrument is as follows. Near-infrared light photons are injected into the skin over the forehead. After being scattered about inside the skin, scalp, skull, and brain, some fraction of the photons survive to return and exit the skin several centimeters away. By measuring the flux of returning photons as a function of wavelength, one can infer the spectral absorptance of the blood in the underlying tissue. For further information, reference is made to Somanetics U.S. Pat. Nos. 4,570,638, 4,725,147, 4,768,516, 4,817,623, 5,139,025, 5,140,989, 5,217,013, 5,349,961, 5,465,714, 5,477,853, 5,482,034, 5,584,296, and 5,697,367. FIG. 1 is a plot of the hemoglobin absorption coefficient as a function of wavelength for hemoglobin having 100% oxygen saturation and hemoglobin having no oxygen.
In order to insure that the photons penetrate deep enough to sample brain tissue, it is necessary to provide a substantial separation between the source and detector optodes. Intuitively, the greater the separation of optodes, the greater the average depth of penetration. Photons that happen to meander close to the surface are very likely to be lost out of the skin before reaching the distant detector. Large optode spacings are therefore biased against xe2x80x9cshallowxe2x80x9d photons, except in the immediate vicinity of the source and detector. On the other hand, increasing path lengths and absorption also make it unlikely that very deeply penetrating photons will find their way back to the detector. Most of the photons reaching the detector will have taken some optimum middle course. It turns out that the average penetration is about 30% of the source/detector spacing.
The unique feature of the Somanetics sensor is that it has two different optode spacings. Data from the smaller, 3 cm spacing (shallower penetration) is subtracted from the larger, 4 cm spacing, which enables suppression of some of the sensitivity to scalp and skull hemoglobin oxygenation while emphasizing the brain oxygenation.
The main drawback to the direct use of NIRS is the requirement for at least two separated optodes (and perhaps a third if scalp oxygenation is to be suppressed) optically connected to the fetal scalp and separated by sufficient distance to insure reflected light has reached the brain. The very considerable practical difficulties of doing this in the hectic birthing environment has discouraged fetal use despite success with the adult head.
Accordingly, it is an aspect of the present invention to solve the above problems by providing an apparatus for monitoring fetal cerebral oxygenation during childbirth using a noninvasive probe that may be positioned on the scalp of a fetus at a single location. To achieve this and other aspects and advantages, the apparatus constructed in accordance with the present invention comprises a source of infrared photons, a probe to be disposed on a single location on the scalp of a fetus for injecting the infrared photons generated by the photon source and for receiving scattered photons reflected from within the skull of the fetus, and first and second detectors for detecting the presence of photons received through the probe at two different delay intervals so as to be capable of distinguishing photons reflected from different depths within the head of the fetus.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.